Field of the Invention
The present invention relates to a liquid ejection head that ejects ink by a liquid ejecting method and records on a recording medium, and a method for cleaning the head. The invention relates also to a recording apparatus provided with the head.
Description of the Related Art
In a liquid ejecting method (i.e., an inkjet recording method), a liquid (e.g., ink) is ejected from ejection ports provided in a liquid ejection head, and is caused to adhere for recording on a recording material, such as a paper sheet. The inkjet recording method in which the liquid is ejected by foaming of the liquid produced by thermal energy generated by electrothermal converting elements enables high quality and high speed printing.
Typically, a liquid ejection head of this kind has a plurality of ejection ports, a flow path communicating with the ejection ports, and a plurality of electrothermal converting elements that generate thermal energy to eject ink. Each of the electrothermal converting elements is constituted by a heat generating resistive element and an electrode that supplies the heat generating resistive element with electric power. The electrothermal converting element is covered with an insulating lower protective layer, such as silicon nitride, and is thus insulated from ink.
A heat generating portion of the electrothermal converting element is exposed to high temperatures and, at the same time, is complexly subject to cavitation impacts caused by foaming and contraction of a liquid, and chemical actions caused by ink. To protect the heat generating resistive element from the cavitation impacts and chemical actions caused by ink, an upper protective layer is provided in the heating unit. A temperature of a surface of the upper protective layer rises to as high as about 700 degrees centigrade and touches the ink. Therefore, the surface needs to be excellent in film characteristics, such as heat resistance, mechanical property, chemical stability, and alkali resistance.
A coloring material, an additive, and other materials included in the ink are separated with a molecular level when heated at high temperatures, and these materials change to hardly soluble materials called “kogation.” When the kogation is physically adsorbed onto the upper protective layer, the following problems occur: heat is conducted unevenly from the heat generating resistive element to the ink, and, therefore, ejection speed of the ink is lowered, foaming becomes unstable, and more energy is required for the ejection.
Then, Japanese Patent Laid-Open No. 2008-105364 discloses a technique to remove kogation by forming a surface of an upper protective layer using a material elutable by an electrochemical reaction, such as iridium and ruthenium.